As users experience the convenience of wireless connectivity, they are demanding increasing support. Typical applications over wireless networks address include video streaming, video conferencing, distance learning, etc. Because wireless bandwidth availability is restricted, quality of service (QoS) management is increasingly important in 802.11 networks. IEEE 802.11e proposes to define QoS mechanisms for wireless gear that gives support to bandwidth-sensitive applications such as voice and video.
The original 802.11 media access control (MAC) protocol was designed with two modes of communication for wireless stations. The first mode, Distributed Coordination Function (DCF), is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), sometimes referred to as “listen before talk.” A station waits for a quiet period on the network and then begins to transmit data and detect collisions. The second mode, Point Coordination Function (PCF), supports time-sensitive traffic flows. Wireless access points periodically send beacon frames to communicate network identification and management parameters specific to the wireless network. Between sending beacon frames, PCF splits the time into a contention-free period and a contention period. A station using PCF transmits data during contention-free periods.
Because DCF and PCF do not differentiate between traffic types or sources, IEEE proposed enhancements to both coordination modes to facilitate QoS. These changes are intended to fulfill critical service requirements while maintaining backward-compatibility with current 802.11 standards.
Enhanced Distribution Coordination Access (EDCA) introduces the concept of traffic categories. Using EDCA, stations try to send data after detecting that the medium is idle for a set time period defined by the corresponding traffic category. A higher-priority traffic category will have a shorter wait time than a lower-priority traffic category. While no guarantees of service are provided, EDCA establishes a probabilistic priority mechanism to allocate bandwidth based on traffic categories.
The IEEE 802.11e EDCA standard provides QoS differentiation by grouping traffic into four access classes (ACs), i.e. voice, video, best effort and background. Each frame from the upper layers bears a priority value (0-7), which is passed down to the MAC layer. Based on the priority value, the frames are mapped into the four ACs at the MAC layer. The voice AC has the highest priority; the video AC has the second highest priority; the best effort AC has the third highest priority; and the background AC has the lowest priority. Each AC has its own transmission queue and its own set of medium access parameters. Traffic prioritization uses the medium access parameters—AIFS interval, contention window (CW), and transfer opportunity (TXOP)—to ensure that a higher priority AC has relatively more medium access opportunity than a lower priority AC.
Generally, the arbitration interframe space (AIFS) is the time interval that a station must sense the medium to be idle before invoking a backoff or transmission. A higher priority AC uses a smaller AIFS interval. The contention window (CW, CWmin and CWmax) indicates the number of backoff time slots until the station can access the medium. CW starts from CWmin and doubles every time a transmission fails until it reaches its maximum value CWmax. Then, CW holds its maximum value until the transmission exceeds its retry limit. A higher priority AC uses smaller CWmin and CWmax. The Transmission Opportunity (TXOP) indicates the maximum duration that an AC can be allowed to transmit frames after acquiring access to the medium. To save contention overhead, multiple frames can be transmitted within one acquired TXOP without any additional contention, as long as the total transmission time does not exceed the TXOP duration.
To reduce the probability of two stations colliding, because the two stations cannot hear each other, the standard defines a virtual carrier sense mechanism. Before a station initiates a transaction, the station first transmits a short control packet called RTS (Request To Send), which includes the source address, the destination address and the duration of the upcoming transaction (i.e. the data packet and the respective ACK). Then, the destination station responds (if the medium is free) with a response control packet called CTS (Clear to Send), which includes the same duration information. All stations receiving either the RTS and/or the CTS set a virtual carrier sense indicator, i.e., the network allocation vector (NAV), for the given duration, and use the NAV together with the physical carrier sense when sensing the medium. This mechanism reduces the probability of a collision in the receiver area by a station that is “hidden” from the transmitter station to the short duration of the RTS transmission, because the station hears the CTS and “reserves” the medium as busy until the end of the transaction. The duration information in the RTS also protects the transmitter area from collisions during the ACK from stations that are out of range of the acknowledging station. Due to the fact that the RTS and CTS are short frames, the mechanism reduces the overhead of collisions, since these frames are recognized more quickly than if the whole data packet was to be transmitted (assuming the data packet is bigger than RTS). The standard allows for short data packets, i.e., those shorter than an RTS Threshold, to be transmitted without the RTS/CTS transaction.
With these medium access parameters, EDCA works in the following manner: Before a transmitting station can initiate any transmission, the transmitting station must first sense the channel idle (physically and virtually) for at least an AIFS time interval. If the channel is idle after the AIFS interval, the transmitting station invokes a backoff procedure using a backoff counter to count down a random number of backoff time slots. The transmitting station decrements the backoff counter by one as long as the channel is sensed to be idle. Once the backoff counter reaches zero, the transmitting station initiates an RTS transmission and awaits a CTS transmission from the receiving station. If the transmitting station receives a CTS transmission from the receiving station, the transmitting station initiates the transaction. The station can initiate multiple frame transmissions without additional contention as long as the total transmission time does not exceed the TXOP duration.
If the transmitting station senses the channel to be busy at any time during the backoff procedure, the transmitting station suspends its current backoff procedure and freezes its backoff counter until the channel is sensed to be idle for an AIFS interval again. Then, if the channel is still idle, the transmitting station resumes decrementing its remaining backoff counter. After each unsuccessful transmission, CW doubles until CWmax. After a successful transmission, CW to CWmin. The level of QoS control for each AC is determined by the combination of the medium access parameters and the number of competing stations in the network.
The default EDCA parameter values used by non-AP QoS stations (QSTAs) are identified in the table below. A TXOP_Limit value of 0 indicates that a single MAC service data unit (MSDU) or MAC protocol data unit (MPDU), in addition to a possible RTS/CTS exchange or CTS to itself, may be transmitted at any rate for each TXOP.
TABLEDefault EDCA Parameter SetTXOP_Limit (ms)ACCWminCWmaxAIFSNfor 802.11 g PHYAC_BK31102370AC_BE31102330AC_VI153123.008AC_VO71521.504
Although 802.11e enhances the original 802.11 media access control, there are still a number of remaining challenges that need to be addressed to provide comprehensive QoS support. For example, as stated above, multiple frames can be transmitted using one acquired EDCA TXOP without additional contentions as long as the total transmission time does not exceed the EDCA TXOP duration. FIG. 1 illustrates an example prior art timing diagram 100 illustrating multiple transactions during a single TXOP duration (commonly referred to as EDCA Contention Free Burst (CFB) in the 802.11e standard). As shown in this example, during the static TXOP duration 125, a sender 110 is capable of sending three frames, namely, frames 105a-105c, to a receiver 115 (including receiver acknowledgements, namely, ACKs 120a-120c). The TXOP duration is static regardless of the video traffic characteristics. While this may be efficient for constant bit rate (CBR) traffic (which presents constant frame size and inter-arrival time), it may not be efficient for variable bit rate (VBR) traffic. For example, many real-time video applications such as video conferencing and video streaming present VBR traffic with time-varying frame size and inter-arrival time. Without sufficient EDCA TXOP duration, low throughput, high delay and large packet loss due to sender buffer overflow may be experienced. While an arbitrarily large EDCA TXOP duration may help to transmit more video frames, it will likely degrade network capacity due to less admitted stations.
A system and method for increasing QoS support in wireless LANs, especially for VBR transmissions, are needed.